The invention relates to a press, in particular a forming press which works by the method of external high-pressure forming, carries out the forming process with a small number of operating steps by virtue of the proposed arrangement of the driving and forming subassemblies and is distinguished by a compact rigid design. A sheet holder system having high flexibility, along with a low overall height, is also proposed.
External high-pressure forming (EHF), also designated as a hydromechanical drawing method or hydrostatic cold-forming method, has been known for many years. The method is already described in detail in DE 12 40 801 of 1961 and reference is made to this description. The increased use of external high-pressure forming as an inexpensive alternative in small-lot production leads to a growing demand for appropriately equipped forming presses. Developments in hydraulic and electronic control engineering also have a beneficial effect on improvements in method. A multi-purpose press working by external high-pressure forming is disclosed in DE 198 19 950. This machine, designed as a hydraulic press, is also designed additionally for forming by internal high pressure and as a deep-drawing press. For the external high-pressure forming application, following method steps then take place:
1 Insert sheet billet
2 Move the holding-down device downward
3 Move the fluid box upward by means of the working cylinders
4 Apply the holding-down force
5 Move the ram downward
6 Move the working cylinders upward shortly before the ram reaches bottom dead center (BDC)
7 Regulation of the fluid pressure in the fluid box
8 The ram moves to BDC, and engage interlocks
9 The working cylinders move further upward and the holding-down cylinders are displaced
10 By the fluid box being moved further up, the clamped sheet billet is drawn via the die located on the fixed ram
11 The holding-down device is pressed to blocking with the spacers and the top spar
12 The working cylinders increase the closing pressure
13 A fluidic high pressure is built up in the fluid box, and the workpiece contour is produced by means of a platicizing operation
14 Relief of pressure
15 The working cylinders lower the fluid box
16 The interlock is disengaged
17 The ram together with the holding-down device is moved up
18 The workpiece is extracted
Quite apart from the multiplicity of process steps, the press is, of course, not designed optimally for the respective intended use, on the basis of the object of operating it as a universal or multi-purpose press.
The object on which the invention is based is to propose a press of high rigidity for external high-pressure forming, which makes it possible to reduce the process steps and in which the fluid box can be arranged stationarily and which has available a freely selectable and demand-related arrangement of the sheetholder cylinders.
The invention is based on the idea of selecting the arrangement of the fluid cylinders in such a way that a rigid and compact design is ensured as a result of a direct position in the force flux. Since the ram and sheetholder are moved jointly, only one movement cylinder is necessary and the sheetholder cylinders may be designed as straightforward short-stroke cylinders. These short-stroke cylinders are not arranged in a rigid system, but, instead, the number and arrangement of these can be selected optimally for the respective workpieces according to the requirements of the forming process. Corresponding coupleable connections to the power line or lines are provided.
The working cylinders carrying the die are integrated directly in the ram. The counterpressure is regulated in a stationary fluid box, thus making it possible to have fixed pipework, this being a further advantage during high-pressure operation.
The process sequence is as follows:
1 Insert a sheet billet
2 Joint downward movement of the ram and sheetholder
3 Interlocking of the ram with sheetholder by means of engaging spacer pieces
4 Downward movement of the die into the fluid box by means of working cylinders, at the same time
4.1 Pressure regulation in the fluid box
4.2 Regulation of the sheet holding force via short-stroke cylinders
5 Interlocking of the die at bottom dead center
6 Build-up of the maximum sheet holding force via short-stroke cylinders
7 High-pressure build-up in the fluid box and operation of plasticizing the workpiece (calibration)
8 Relief of pressure
9 The interlock is disengaged
10 The ram together with the holding-down device is moved up
11 The workpiece is extracted
The entire process sequence consequently consists of altogether 11 operating steps which also even partially overlap. This also ensures, as a further advantage, a high cycle rate and an economical use of the press.
A favorable energy balance due to small oil quantities being required is achieved as a result of the common movement cylinder for the ram and sheetholder and the use of the short-stroke cylinders for regulating the sheet holding force and for building up the maximum locking force. Since the short-stroke cylinders do not themselves carry out the forming travel, but execute merely a stroke of a few millimeters, an inexpensive design with a low overall height is obtained. This is also advantageous when multi-point support is necessary on account of the workpiece geometry.
It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory and are intended to provide further explanation of the invention as claimed.